Revert replacing generic Ui.OpenWindow with static method in MusicPlayerLogic

OpenRA.Mods.RA/Widgets/Logic/MainMenuLogic.cs

@@ -110,7 +110,10 @@ namespace OpenRA.Mods.RA.Widgets.Logic
 			extrasMenu.Get<ButtonWidget>("MUSIC_BUTTON").OnClick = () =>
 			{
 				menuType = MenuType.None;
-				MusicPlayerLogic.OpenWindow(world, () => menuType = MenuType.Extras);
+				Ui.OpenWindow("MUSIC_PANEL", new WidgetArgs
+				{
+					{ "onExit", () => menuType = MenuType.Extras },
+				});
 			};
 
 			var assetBrowserButton = extrasMenu.GetOrNull<ButtonWidget>("ASSETBROWSER_BUTTON");